Direct type back light unit for liquid crystal display device

ABSTRACT

A back light unit for a liquid crystal display device for improving uniformity of the light and dispensing with optical sheets. The back light unit includes a light source having a holographic pattern formed on a surface thereof opposite the display device which is to display a picture, and a reflective plate under the light source.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/314,338, filed on Dec. 8, 2008, now U.S. Pat. No. 7,802,894, which isa divisional of Ser. No. 11/119,916, filed May 3, 2005, now U.S. Pat.No. 7,473,009, issued on Jan. 6, 2009, which claims the benefit ofKorean Application No. P2004-113793, filed on Dec. 28, 2004, all ofwhich are hereby incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to back light units for a liquid crystaldisplay devices. More particularly, the present invention relates to aback light unit having a holographic pattern layer.

2. Discussion of the Related Art

Active research has been conducted relating to flat display devices. Theresearch has been most focused on Liquid Crystal Display Devices (LCDs),Field Emission Display Devices (FEDs), Electro-luminescence DisplayDevices (ELDs), and Plasma Display Panels (PDP). Of the flat displaydevices, a field of application for liquid crystal display devicesexpands to notebook PCs, desk top monitors, and liquid crystal TVsincluding features such as high contrast ratio, suitability forexpression of gray scales and motion pictures, and low powerconsumption.

However, because the liquid crystal display device is not luminous, theliquid crystal display device requires an external light source foremitting a light. Particularly, in a case of a transmissive liquidcrystal display device, a separate illuminating device for emitting andguiding the light to a back surface of the LCD panel, i.e., a back lightunit, is required, invariably.

Back light units include an edge type and direct type based upon themethods of projecting the light. The edge type back light unit has atube-type linear light source, such as a lamp light, at a side of theliquid crystal panel, to project the light from the lamp light to theliquid crystal panel throughout an entire surface thereof. The directtype back light unit has lamp lights selectively mounted under theliquid crystal panel to distribute the light from the lamp lights to theliquid crystal panel throughout an entire surface thereof, by diffusingthe light by means of a diffusion sheet located between the lamp lightsand the liquid crystal panel.

Since no light plate is required, the direct type back light unit isadvantageous in that the direct type back light unit is suitable forfabrication of a thin, and light weight liquid crystal display devicethat provides a high luminance and uniform light distribution.

A related art direct type back light will be described with reference tothe attached drawings.

FIG. 1 illustrates an exploded perspective view of a related art directtype back light unit, and FIG. 2 illustrates a section across I-I′ ofFIG. 1, and FIGS. 3A and 3B illustrate SEM photographs of diffusionsheets.

A liquid crystal display device having the back light unit as a basicelement will be described.

Generally, the liquid crystal display device includes a liquid crystalpanel having opposite upper and lower substrates bonded together with aliquid crystal layer inbetween, polarizing plates respectively attachedto upper, and lower surfaces of the liquid crystal panel fortransmitting light of only one direction, a direct type back lightmounted under the liquid crystal panel for providing a light to theliquid crystal panel, a case for covering an outside surface of the backlight unit to support the liquid crystal panel and the direct type backlight unit, and a bezel part of stainless steel attached to an outsideof the case for covering a circumference of the liquid crystal panelexcept an effective area where a picture is to be displayed thereon.

In FIGS. 1 and 2, the direct type back light is provided with a lamplight 111 for providing a light, and on optical sheet having a diffusionsheet 115, a prism sheet 116, and a protective sheet 117 between thelight lamp 111 and the liquid crystal panel 110.

In general, there is a reflective plate (not shown) under the light lamp111 for directing the light from the light lamp 111 toward the displayportion of the liquid crystal panel.

The lamp light 111 may be a flat lamp light or a Cold CathodeFluorescent Lamp (CCFL).

The flat lamp light has a discharge space formed by upper and lowerboards sealed with a gap therebetween with discharge gas, such as neonNe, argon Ar, or mercury Hg filled therein, a fluorescent material layercoated on each of opposite inside wall surfaces of the upper and lowerboards, and an electrode formed on the lower board for applying powerthereto.

Since a certain space, and adequate optical sheets are required betweenthe lamp light and the liquid crystal panel for preventing a shape ofthe lamp light from being visible on a screen, use of the CCFL imposes alimitation on fabrication of a thin liquid crystal display device.Therefore, in a case when a light source with a comparatively largedisplay area and a uniform luminance is required, the flat lamp lightwill be more suitable, of which an entire surface opposite to a displayface of the liquid crystal panel is luminous.

The diffusion sheet 115, the prism sheet 116, and the protective sheet117 are collectively called the optical sheet. There are a plurality ofoptical sheets between the light lamp 111 and the liquid crystal panel110 to enhance a light diffusing effect to prevent the shape of thelight lamp from being visible on the display surface of the liquidcrystal panel and provide a light having a uniform luminancedistribution throughout the surface.

In detail, the diffusion sheet 115 uniformly diffuses the light from thelamp light, the prism sheet 116, with a plurality of triangular linearprisms, collects the diffused light, and directs the light toward theliquid crystal panel, and the protective sheet 117 on the prism sheet116 protects the prism sheet 116. As shown in FIGS. 3 and 4, thediffusion sheet 115 has spherical light diffusing fine particles 115 aon a surface for diffusing the light by differences of refractiveindices.

As the light diffusing fine particles 115 a, glass, polystyrene,polycarbonate, PMMA, or so on is used, which makes a light transmits,and refracts, to change a light path, and diffuse the light. The prismsheet 116 has spherical or non-spherical lenses added to a prism sheetwhich can collect a diffused light, for collimating the light within apredetermined angle for improving a luminance.

However, the placing of various kinds of optical sheets between theliquid crystal panel and the lamp light impairs an optical efficiency asa portion of the light from the lamp is absorbed in the optical sheetsas the light passes through the optical sheets, the stacking and lightcombination of various kinds of optical sheets may cause a defect inview of fabrication process, and the increased thickness of the sheetsis contrary to development of a thin liquid crystal display device whichconsumers require.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a direct type backlight unit for a liquid crystal display device that substantiallyobviates one or more problems due to limitations and disadvantages ofthe related art.

An advantage of the present invention is to provide a back light unitfor a liquid crystal display device in which a holographic pattern thatchanges the direction and distribution of a light is formed on a lamplight to improve uniformity of the light, reduces or removes the needfor optical sheets to fabricate a thin liquid crystal display device.

Additional advantages and features of the invention will be set forth inpart in the description which follows and in part will become apparentto those having ordinary skill in the art upon examination of thefollowing or may be learned from practice of the invention. Theadvantages of the invention may be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, a back lightunit for a liquid crystal display device includes a light source and aholographic pattern on a light path of a light from the light source.

The holographic pattern controls the direction and distribution of thelight incident thereon.

For example, the light source may be a flat lamp light, and theholographic pattern may be formed on a surface of one side of the flatlamp light, or the holographic pattern may be positioned between theflat lamp light and the liquid crystal panel.

The light source includes a plurality of CCFLs, and a glass board on theplurality of CCFLs. The holographic pattern is formed on the surface ofthe glass board.

The holographic pattern is formed by casting glass in a mold having aholographic image recorded thereon, or by applying an optical refractivematerial on an upper surface of the flat light lamp, and directing alaser beam thereto.

The back light may be a direct type or of an edge type.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings;

FIG. 1 illustrates a perspective view of a related art direct type backlight unit;

FIG. 2 illustrates a section across a line I-I′ in FIG. 1;

FIGS. 3A and 3B illustrate SEM photographs of diffusion sheets,respectively;

FIG. 4 illustrates a plan view of the direct type back light inaccordance with an embodiment of the present invention;

FIG. 5 illustrates a section across a line II-II′ in FIG. 4; and

FIGS. 6A and 6B illustrate SEM photographs of holographic pattern inaccordance with a preferred embodiment of the present invention,respectively.

FIG. 7 illustrates a section view of a direct type back light unit for aliquid crystal display device in accordance with another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 4 illustrates a plan view of a direct type back light in accordancewith an embodiment of the present invention, FIG. 5 illustrates asection across a line II-II′ in FIG. 4, and FIGS. 6A and 6B illustrateSEM photographs of holographic patterns in accordance with an embodimentof the present invention, respectively.

In FIGS. 4 and 5, the direct type back light flat lamp light includes anupper board 21 having a holographic pattern 11 formed on an outsidesurface, a lower board 21 a opposite the upper board 21 with a gaptherebetween, side boards 29 each placed between edges of the upper andlower boards 21 and 21 a, respectively, to seal the upper and lowerboards 21 and 21 a, supporting rods 22 between the upper board 21 andthe lower board 21 a for supporting the upper board 21, at least onefirst electrode 23 inserted in the side board 29, and at least onesecond electrode 23 a opposite the first electrode 23.

The space between the upper and lower boards 21 and 21 a are sealed withthe side boards 29 to form a discharge space 26 having discharge gas,such as neon Ne, argon Ar or mercury Hg filled therein.

The first electrode 23 and the second electrode 23 a are completelysealed to the side boards 29 with a bonding material, and have endsconnected to power leading lines (not shown), for applying an externalvoltage thereto.

In order to prevent a visible light generated in the discharge spacefrom escaping through a back surface of the lower board 21 a, either areflective plate (not shown) provided under the lower board 21 a, or amaterial having a reflective property may be coated on an inside wall ofthe lower board 21 a.

The flat lamp light generates a visible light when the discharge gas,which induces discharge if an appropriate voltage is applied to thefirst and second electrodes 23 and 23 a, starts to discharge, i.e.,electrons moving by the discharge collide with other electrons togenerate an UV ray. The UV ray hits the fluorescent material layer 28 onthe inside walls of the upper and lower boards 21 and 21 a to excite andrestore the fluorescent material to generate the visible light.

In this instance, the visible light passes through the holographicpattern 11 on the upper board 21 to diffuse and collimate at the sametime. The visible light is then provided to the liquid crystal panel 10.

The holographic pattern 11 is formed to diffuse the light in apredetermined direction and angle regardless of an incident angle of thelight. Therefore, a direction and distribution of the light can becontrolled by using the holographic pattern 11 to collimate or diffusethe light.

That is, even if the light from the lamp light proceeds in a straightand upwardly direction, once the light passes through the holographicpattern, because the light diffuses within a range of angle withreference to a vertical axis of the holographic pattern regardless of anincident angle of the light, the direction and distribution of the lightcan be controlled.

Thus, since the direct type back light of the present invention diffusesas well as collimates by means of the holographic pattern, no separateoptical sheets are required. However, in a case of a large sized liquidcrystal display, optical sheets may be used to provide a more uniformlight; thus, the number of the optical sheets can be reduced to aminimum.

In general, the upper board 21 of the flat lamp light is constructed ofa transparent, heat resistant material board, for example, glass. Theholographic pattern is formed by casting the transparent material boardin a mold having a holographic image formed therein to pattern theholographic image on the transparent board.

In order to form the holographic image in the mold, a method of exposingphotoresist having a fine holographic lattice, a method of directcarving with a laser beam or an e-beam, a dry etching method, a diamondturning method, or so on is used.

In FIGS. 6A and 6B, the holographic pattern formed using one of theabove methods has semi-spherical shapes of a variety of sizes and shapesof irregular patterns to enable diffusing and collimating of the lightin various directions. Therefore, by controlling shapes and sizes of theholographic patterns, a direction and a distribution of the light can becontrolled.

As an example, a small pattern used between large patterns may transmitand refract a light passing between the large patterns to enhance alight transmission efficiency, as well as improve a light diffusingfunction in order to obtain a uniform light source.

For reference, the collimating and diffusing of the light occurs byrefraction of the light based upon refractive indices of media. If thelight is incident on a boundary surface of two media slanted withrespect to the boundary surface, the light refracts to a directionvertical to the boundary surface if the refractive index of the secondmedium is great. The light can be collimated or diffused by thisprinciple.

In another embodiment of the present invention shown in FIG. 7, insteadof the flat lamp light, a plurality of CCFLs or EEFLs 32 may be used asthe light source. In this case, a transparent board 31, for example aglass board, may be placed on the plurality of CCFLs or EEFLs 32 and aholographic pattern 11 is formed on the glass board 31.

In another embodiment of the present invention, an optical refractivematerial, such as dichromate gelatin, photoresist, photopolymer, silverhalide, or so on may be coated on the upper surface of the lamp light,and a laser beam may be directed thereon to form an interference patternof a speckle shape in order to form the holographic pattern. Since thesmaller the speckle shaped interference pattern, the more the lighttransmitting through the holographic pattern diffuses, and the greaterthe range of diffusing angle of the light, the light distribution andthe like can be controlled by controlling a size of the holographicpattern.

In another embodiment of the present invention, instead of the flat lamplight, a plurality of CCFLs or EEFLs may be used as the light source. Inthis case, a transparent board, for example a glass board, may be placedon the plurality of CCFLs or EEFLs and a holographic pattern is formedon the glass board.

As has been described, the back light unit for a liquid crystal displaydevice of the present invention has the following advantages.

First, control of light distribution and the like by means of aholographic pattern on a surface of a lamp light opposite to a liquidcrystal panel permits improve light uniformity.

Second, collimating and diffusing of a light by means of a holographicpattern permits one to dispense with, or reduce a number of opticalsheets, which permits the fabrication of a thinner liquid crystaldisplay device.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A back light unit for a liquid crystal display device comprising: alight source including a plurality of lamps a transparent glass board onthe plurality of lamps; and a holographic pattern is directly formed ona surface of the transparent glass board, wherein the holographicpattern is formed by casting a transparent material in a mold having aholographic image recorded thereon, and wherein the holographic patternhas semi-spherical shapes of a variety of sizes and shapes of irregularpatterns to enable diffusing and collimating of a light in variousdirections, and wherein the holographic pattern includes large patternsand small patterns between the large pattern, wherein the small patternused between the large patterns transmit and refract the light passingbetween the large patterns.
 2. The back light unit according to claim 1,wherein the plurality of lamps are CCFLs or EEFLs.
 3. A method offorming a liquid crystal display device, the method comprising:providing a light source including a plurality of lamps; providing atransparent glass board on the plurality of lamps; and providing aholographic pattern is directly formed on a surface of the transparentglass board, wherein the holographic pattern is formed by casting atransparent material in a mold having a holographic image recordedthereon, and wherein the holographic pattern has semi-spherical shapesof a variety of sizes and shapes of irregular patterns to enablediffusing and collimating of a light in various directions, and whereinthe holographic pattern includes large patterns and small patternsbetween the large pattern, wherein the small pattern used between thelarge patterns transmit and refract the light passing between the largepatterns.
 4. The method according to claim 3, wherein the plurality oflamps are CCFLs or EEFLs.